1. Technical Field
The present invention relates to sealing high strength monolithic carbon foams useful for applications including as composite material tooling. More particularly, the present invention relates to a process and material for sealing the surface of blocks of carbon foams to provide a working surface thereon or to permit surfaces of two blocks of foam to be cemented together to form larger blocks. A sealed carbon foam is also provided.
2. Background Art
Carbon foams have attracted considerable recent activity because of their properties of low density, coupled with either very high or low thermal conductivity. Indeed, in their U.S. Pat. No. 6,849,098, Joseph and Rogers describe the use of a carbon foam product as tooling for the production of composite materials. Unfortunately, the foams produced by the processes of the Joseph and Rogers patent are unsuitable for use as tooling without the addition of a layer of a so-called facesheet material, because of the pore structure of the foams, including the open-celled nature with highly interconnected pores. These facesheet materials are used to either completely or partially fill the foam cell volume in the Joseph and Rogers disclosure. However, filling of cells with these facesheet materials will raise the density and reduce the effectiveness of the foam as a lightweight material for use in composite tooling. In addition, the use of these facesheets creates difficulties in adhesion and in terms of lack of thermal expansion compatibility. Also, the Joseph and Rogers foam products likely do not have the required strength to density ratios needed for tooling and other structural applications, and the foams produced in accordance with the Joseph and Rogers techniques are not available in sufficiently large blocks for tooling, thus requiring several blocks to be cemented together; while cementing of blocks together can and is successfully done, the more cemented joints that are present, the less structural integrity the resulting block will have.
Recently, a carbon foam has been developed and commercialized under the trademark GRAFOAM by UCAR Carbon Company Inc. of Wilmington, Del. This foam is monolithic and has a controllable cell structure, where the cell structure, strength and strength to density ratio make the foam suitable for use as composite tooling as well as in other applications. Indeed, a combination of characteristics found in GRAFOAM carbon foam, including strength to density ratios higher than contemplated in the prior art, have been found to be necessary for use of a carbon foam in composite tooling applications.
While such carbon foams have the pore structure to provide low gas permeability in comparison to other open-cell carbon foams available in the market, the open porosity on the carbon foam surface still needs to be sealed in order to make the carbon foam useful for many applications such as tooling. Using carbon foam as core material for sandwiched composite structure also requires sealing. If the foam surface is not adequately sealed, resin will infiltrate into the foam during composite manufacturing processes such as resin infusion and vacuum assisted resin transfer molding.
A carbon foam suitable for applications such as composite tooling has two distinct pore size distributions. One pore size is in the micron meter range; the other in the tens to hundreds micron meters range. The foam has a largely closed cell structure with low interconnectivity. Conventional commercial sealers, depending upon the viscosity, cannot seal the pores. In the case of low-viscosity commercial sealers, sealers will wick into the foam and cannot seal the surface even after many applications. In the case of high-viscosity commercial sealers/adhesives such as those commercially available as Loctite 9394 and 9396, it is found that the sealer skin shrinks during curing, resulting in delamination.
Cements for carbon bodies have been disclosed in the past. For instance, in U.S. Pat. No. 5,002,981, Chiu discloses a carbonaceous cement paste composition having carbonaceous particles and a resin binder system composed of a high-temperature polymeric resin binder and a thermosettable furfuryl alcohol for dissolving the resin, and a heat-activated catalyst for effecting thermosetting of the furfuryl alcohol upon heating. The carbonaceous particles disclosed by Chiu include graphite flour, coke flour, carbon black, pitch coke flour and calcined lampblack flour. The carbonaceous particles are present at from about 20% to about 85% by weight.
Likewise, in U.S. Pat. No. 6,214,158 to Chiu, Lewis and Lewis, a cement paste composition is disclosed, including a catalyst; a carbon filler present in an amount of about 20 to about 60% by weight; a polymerizable monomeric system; and a furan solvent. The curable cement composition may be used along with a pre-coat in an adhesive system for attaching together carbon bodies. The carbon filler can be pitch coke flour, petroleum coke flour, graphite powder, coal, carbon black or mixtures thereof
In U.S. Pat. No. 5,280,063, Lewis, Pirro, Greinke, Bretz and Kampe disclose a room temperature setting carbonaceous cement comprising a solid carbonaceous material, a catalyst and a liquid carbonizable component which when treated with the catalyst will provide a carbon yield of at least 40% at an elevated baking temperature and possess a flexural strength which is above at least 750 psi at room temperature and at said elevated temperature.
Tyler, in his U.S. Pat. No. 3,441,529, discloses a thermosetting cement for bonding carbonaceous structures, having a mixture of an oil, a soap, finely-divided carbonaceous particles, furfuryl alcohol, a phenolic novolac resin, and a hardening agent.
Although effective as cements, there is no disclosure in the prior art cement patents of the use of the disclosed cements to either seal carbon foams or bond together blocks of carbon foams, especially those having a pore structure uniquely suited for use in applications such as composite tooling. Carbon foam has an open-celled structure that cannot be sealed easily like a monolithic graphite or other “solid” carbon block (used for tooling).
What is desired therefore, is a material useful for sealing carbon foams, and the sealed carbon foams themselves, especially carbon foams whose pore structure, strength, and strength to density ratio is suitable for use in application such as composite tooling.